docs: def label_count(self, label_list_ids=None):
          """
          Return a dictionary containing the number of times,
          every label-value in this utterance is occurring.
  
          Args:
              label_list_ids (list): If not None, only labels from label-lists
                                     with an id contained in this list
                                     are considered.
  
          Returns:
              dict: A dictionary containing the number of occurrences
                    with the label-value as key.
          """
          count = collections.defaultdict(int)
  
          for label_list in self.label_lists.values():
              if label_list_ids is None or label_list.idx in label_list_ids:
                  for label_value, label_count in label_list.label_count().items():
                      count[label_value] += label_count
  
          return count